The most oxygen-poor planetary nebula: AGB nucleosynthesis at low metallicities

TS01 is an exceptional planetary nebula (PN) in the Galactic halo: it is the most oxygen-poor and has a double-degenerate core with mass close to 1.4M_⊙, possibly a Supernova Ia progenitor. With data from the far UV to the IR we can pin down the abundances of half a dozen of elements. The oxygen abundance is by 1.9(±0.3) dex lower than in the Sun. Standard AGB models with appropriate mass and metallicity cannot explain the observed chemical composition. We find that additional mixing, induced by stellar rotation and/or by the presence of the close companion can explain most of the features of the abundance pattern in TS01.